Laser apparatuses such as these are known, for example, from the documents WO 01/93386 and WO 02/067393, the contents of which are incorporated in the present description by reference. These documents describe a surface-emitting semiconductor laser apparatus, which contains a laterally optically pumped quantum well structure. An edge-emitting semiconductor laser is used, by way of example, as the pump radiation source for this quantum well structure. The pump radiation is guided in a waveguide within the pump radiation source. The pump radiation source and the quantum well structure are each formed by means of a semiconductor layer sequence, with the semiconductor layer sequences being grown epitaxially on a common substrate. This allows low-cost monolithic integration of the quantum well structure and the pump radiation source.
In principle, an increase in the optical output power can be achieved by enlarging the optically pumped volume. To do this, it is desirable to increase the lateral diameter of the optically pumped structure, in which case fundamental mode operation would preferably be possible, or should preferably be retained.
However, merely enlarging the lateral diameter leads to problems, which will be explained in the following text with reference to FIG. 8:
FIG. 8 shows an optically pumped semiconductor laser apparatus having a vertical emitter 20, which has a radiation-producing structure with two or more quantum layers 21. The radiation-producing structure may be surrounded by mirrors 25 on one side or both sides in order to form a vertical laser resonator. Two pump lasers 22 are arranged laterally adjacent, in order to produce the pump radiation. The pump lasers 22 each have a waveguide 23, in which the pump radiation is guided.
In known apparatuses of this type, these waveguides are arranged at the same height as the quantum layers 21, in order to efficiently inject the pump radiation into the radiation-producing structure. In particular, the quantum layers 21 are aligned centrally with the pump waveguide 23 and are formed as close as possible to its center axis 26, so that the spatial maximum of the pump field 24 precisely overlaps the quantum layers of the vertical emitter. This requires very precise manufacture.
If the lateral diameter of the vertical emitter is enlarged, it is evident that only a very restricted increase in the optical output power is possible by this enlargement on its own or in conjunction with a higher pump power. This is because the quantum layers strongly absorb the pump radiation when the pump radiation is aligned precisely with the quantum layers. The pump radiation is thus absorbed in particular in the vicinity of the boundary surface between the pump laser and the vertical emitter, thus resulting in a short lateral penetration depth. Beyond this lateral penetration depth, the vertical emitter is scarcely pumped at all. Laser operation is thus achieved only at the edge of the vertical emitter, in a comparatively small area, which is typically narrower than 30 μm.
Enlarging the lateral diameter or increasing the pump power does not change the penetration depth, so that the optical output power is increased only slightly in this way. In particular, this does not result in an increase in the output power as a result of enlargement of the pumped active area.
Furthermore, pumping of the quantum layers in the vicinity of the boundary surface between the pump laser and the vertical emitter is associated with additional losses, since the charge carriers which are produced during the pumping process diffuse within a diffusion length of about 10 μm with respect to the boundary surface, where they can recombine, without emitting radiation, as a result of boundary surface defects. For this reason as well, it is desirable to enlarge the lateral diameter of the vertical emitter in order to pump the quantum layers in this way as far as possible away from this boundary surface. However, once again, merely enlarging the lateral diameter is not sufficient, since this does not change the absorption close to the edge within a comparatively short penetration depth.